Machining an AR-15 upper receiver forging – part III

Step 3, Drilling the lugs.

We’re finally going to create the actual reference point – the pivot pin hole in the front lug.  While this could be done in a vise, using a 1-2-3 block on the bottom of the receiver and indicating the sides (upward facing surfaces) to the same depth, I decided a fixture would be easier.  The lug drilling plate (see Appendix A) aligns the lugs with the bottom of the vise or angle plate and allows for moderately quick fixturing.  I recommend it if you’re going to make multiples.

Receiver clamped in using lug drilling plate. The centerfinder has just been used to find the 'deck' surface.

Receiver clamped in using lug drilling plate. The edge-finder has just been used to find the inside of the front lug.

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Machining an AR-15 upper receiver forging – part II

Setup 2, the deck

Next we’ll machine the deck flat and cut the lugs to size.  In doing so, we’ll establish a reference to the position of the upper along the line established in the first setup.  The real reference will be the pivot pin hole once we have machined enough that we can drill it accurately.

We’ll use the convention that the X dimension goes more positive as the table moves from left to right (as if the cutter were moving right to left) and that the Y dimension increases as the table moves away from you.

Setup the part, using the clamping plate, with the deck up and the thread boss pointed left. Make sure the deck is about 1/8″ above the top of the angle plate, so you don’t cut into it. Indicate the deck to be as horizontal as possible.  It’s still a raw forging at this point, so the surface won’t be perfect – just get it as close as possible.  Less than 0.010″ across the space between the lugs is good.  Dont indicate along the parting line, use the smooth surface to one side or another.

Indicating the deck to horizontal.

Indicating the deck to horizontal.

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Machining AR-15 upper receiver forgings part I – redux

Machining the upper receiver of an AR-15 is an intermediate to advanced machining project.  It is more difficult to make, on manual machines, than the lower receiver, and should only be undertaken by those who already possess a firm grasp of machining fundamentals.  This document is based on the format of the “Ray-Vin” (Ray Brandes) document which describes the machining of a lower receiver forging.

Because of the complexity, a grasp of machining techniques and terminology are assumed.  This tutorial is not intended as a machining lesson for the beginner.

I freely admit that I am not a professional machinist and that there are likely better ways to perform certain operations.  The setups shown reflect my preferences and the tooling on hand in my shop.  There’s more than one way to skin a dog, and you should feel free to modify the instructions to suit your equipment and preferences.

If you have never machined from a forging or casting before, things are a little different than machining from billet. The AR upper receiver forging has no truly flat or truly round surfaces to use as a starting reference, so you must establish references as early in the machining process as possible and then use them throughout.  You have not created complete references until you have points you can measure from to achieve consistent location in the X, Y, and Z planes, as well as theta (rotation).  Don’t worry too much if that doesn’t make sense to you – we’ll fill in the blanks as we go along.

Setup 1, Establishing theta.

In this first setup, we begin the process of establishing our references.  The first reference point we want to create is the centerline of the bore.   This will give us the centerline of the forging.  The best way to create this reference is to use the thread boss at the front of the receiver.  The thread boss is the round ‘nose’ of the forging.

Dial in an angle plate to be parallel to the X-axis on the mill.

Mount the forging vertically using the receiver mounting plate.  Using the quill to run up and down in the Z-axis, adjust the forging to be vertical.  Note that the forging doesn’t have an accurate edge, so you are adjusting for minimum deflection of the indicator, no zero deflection.  Up to 0.010″ is normal, and nothing to be concerned with.  Using a machinist’s square can help speed this up.

Dialing the receiver in to vertical.  The surface isn't perfectly flat, but it's pretty close

Dialing the receiver in to vertical. The surface isn’t perfectly flat, but it’s pretty close

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Machining AR-15 upper forgings

It takes so long to generate a meaningful post (complete project) that we’re going to try posting progress pics.  Consider these a teaser.  When the project is done, I’ll publish a complete tutorial, with dimensions.  Until then, here’s what we’re working on…

We got some AR-15 upper receiver forgings (both ‘A2 and ‘A3 style).  After quite a bit of planning (these are more difficult than the lower), we’ve started making chips.

Aligning the first setup

Aligning the first setup

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Wednesday Book Review: Hatcher’s Notebook

Hatcher's Notebook

Hatcher’s Notebook


Julian Hatcher (1888-1963) is about as closely aligned with firearms as a person could be.  Raised in the same time period that saw the innovations of pump-action, lever action, and self-loading rifles and pistols and machine guns, his life spanned a period that encompassed the greatest advances in firearms.  He held positions in Army Ordnance, the NRA, and numerous competitive shooting organizations as well as writing and editing for several firearms periodicals.

Perhaps most notably, he was the Officer In Charge of the Springfield Armory Experimental department and participated in numerous studies and experiments spanning a broad range of firearms design related subjects.

During these years, he kept notes, and these were eventually compiled into a book Hatcher’s Notebook – a veritable cornucopia of information about firearms, designs of the times, and experiments that most of us would find impractical to conduct given that we don’t have the financial backing of the government.

In 1957, he updated Hatcher’s Notebook to include some new material.  That work, the 1957 edition of Hatcher’s Notebook, is a goldmine of firearms history, design detail, and observation that is unlikely to ever be reproduced in the public or private sector.

It is nearly impossible to review Hatcher’s Notebook without providing a laundry list of material covered as the topics explored range broadly across virtually all aspects of firearms.  Among the items mentioned on the front cover are: automatic mechanisms, machine guns and semi-automatics, barrel obstructions, rifle strengths and weaknesses, exterior ballistics, recoil problems, headspace, triggers, caliber equivalents, range, and gunpowder.

I know of no other firearms manual which so directly addresses the broad study of firearms design.  It is, however, an old manual and suffers slightly in some regards.  Hatcher’s discussion of ‘spin drift’, for example, contains conclusions that are now considered erroneous.  Fortunately such errors are few and far between.

This is a book that undoubtedly belongs on the shelf of anyone engaged in firearms design, but it is not perfect.

4.5 stars

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